1. Field of the Invention
The present invention relates to a method for realizing a primary photometric standard of optical radiation and a photodetecting apparatus therefor, more particularly, to a method for realizing a primary photometric standard of a photometric unit such as cd, 1m using a photodetector, and a photodetecting apparatus therefor.
2. Description of Related Art
Conventionally, the primary standard of the photometric units is realized by using a black body furnace made based on the Planck's law of radiation, an absolute radiometer for measuring an optical power by substituting the optical power with an electric power, or a synchrotron. In these methods, in order to measure an optical power with an accuracy high enough for a primary standard, it is necessary to provide a large-scale equipment and a high technique, and it can be performed only in the national research organizations.
However, recently, there has been developed a method for measuring an absolute responsivity of a semiconductive photodetector such as a silicon photodiode with a high precision by a relatively simple technique based on a physical study of the semiconductor (See a reference : Applied Optics Vol. 19, No. 8, 1980, pp1214). This method is called "the self-calibration method using a silicon photodiode".
In the self-calibration method using the silicon photodiode, an absolute responsivity [A/W] of the silicon photodiode is obtained by measuring the surface reflectance and the internal quantum efficiency of the silicon photodiode, and it is confirmed that a high accuracy in the order of 0.1% is obtained by this method using a comparatively simple equipment. It has been supposed that the aforementioned self-calibration method using the silicon photodiode is promising as a new method for realizing the primary photometric standard of optical radiation.
However, since the conventional self-calibration method is fundamentally the method for obtaining the absolute responsivity [A/W] at one wavelength of a monochromatic light by projecting the monochromatic light onto the silicon photodiode, in order to realize the photometric standard, it is necessary to measure the absolute responsivity R(.lambda.) over the entire visible region using the self-calibration method. After measuring the absolute responsivity R(.lambda.), there can be calculated the responsivity R.sub.LF [A/1m] for the photometric quantity using the following equation by combination of the silicon photodiode and an optical filter for the spectral luminous efficacy correction whose spectral transmittance .tau.(.lambda.) has been accurately measured, resulting in a photometric standard. ##EQU1##
where P(.lambda.) is a relative spectral distribution of a light source to be measured.
V(.lambda.) is a standard spectral luminous efficiency, and
K.sub.m is a maximum spectral luminous efficacy (683 [1m/W]).
Further, when a photodetecting apparatus for realizing a primary photometric standard of optical radiation (referred to as a photometric standard photodetecting apparatus hereinafter) is constituted by combining the combination of the aforementioned photodiode and the filter for the spectral luminous efficacy correction with an aperture having an aperture dimension Aa [m.sup.2 ], an responsivity [A/1x] for a photometric quantity (1x] can be calculated using the following equation: ##EQU2##
Thereafter, a photometric standard of optical standard can be realized by calculating an illuminance of the photodiode on a photodetecting surface thereof.
As described above, the absolute responsivity can be calculated with a high precision of an order of 0.1% by the self-calibration method, however, it is necessary to measure the spectral transmittance .tau.(.lambda.) of the filter for the spectral luminous efficacy correction with a high precision similar to that of the absolute responsivity since the error in the measured spectral transmittance .tau.(.lambda.) thereof propagates to the calculated absolute responsivity, directly. Generally, the spectral transmittance .tau.(.lambda.) can be measured easily using a spectrophotometer, however, the precision of the measurement generally is only about 1%. In order to measure the spectral transmittance .tau.(.lambda.) of the optical filter with a higher precision, it is necessary to provide another extra measuring instrument.
Further, generally, there is nonuniformity of the transmittance depending on the position on the surface of the optical filter. Therefore, it is necessary to measure the transmittance of the exact portion of the optical filter through which the incident light actually passes when the optical filter is incorporated in the photodetecting apparatus. Otherwise, there may be caused an error in the measured transmittance of the optical filter. Since the diameter of the beam is predetermined in a general spectrophotometer, it is extremely difficult to measure the spectral transmittance of the optical filter under the aforementioned conditions.
Further, even if the spectral transmittance thereof is measured with a high precision taking the aforementioned conditions into considerations, there is such a problem that an apparent transmittance of the optical filter for the spectral luminous efficacy correction increases due to an interreflection caused between the photodetecting surface of the photodiode and the optical filter when the optical filter is mounted in the photometric standard photodetecting apparatus, resulting in an error in the calculated absolute responsivity for the photometric quantity.